I ran into the same problem this weekend. I own a N800 and tried to install the latest fennec which list under App Manager requiring 12 MB but fails to mention that it requires to install Xul Runner which was 30 MB is size.

My App Manager would not update and list the applications I had installed. So I popped out the battery hoping a hard reset would do the trick. In my case it did. The applications showed up again and I was able to uninstall fennec.

It may not be the logical solution but it did work for me. Although if it didn't work, I would've start blowing apps away left, right and center.